One of the commonly used methods for joining thermoplastic parts is ultrasonic welding, which involves melting of the mating part surfaces by means of high-frequency, small-amplitude vibrations while the parts are pressed together. When a sufficient amount of melting has occurred, the ultrasonic vibrations are terminated and the plastic solidifies while the compressive force on the parts is maintained, producing a permanent assembly. Ultrasonic welding offers a number of advantages over other joining methods, including speed, flexibility, cleanliness, and low cost.
A key consideration in the successful use of ultrasonic welding is the design of the weld joint; namely, the geometries of those areas of the parts to be joined which are melted during the welding process. A number of different weld joint designs are known to those skilled in the art, each suited for the purpose of meeting specific weld criteria or to facilitate welding of certain materials.
One of the most commonly used weld joint designs is the triangular energy director, consisting of a ridge of material having a triangular profile on one of the parts and a flat surface on the mating part. This joint design is illustrated in the example of FIGS. 1-3. FIG. 1 shows the two parts, with part 1 containing the energy director 1a and part 2 containing the flat surface. FIG. 2 is a cross-sectional view of the parts in position for welding, where the tip of the energy director of part 1 is in contact with the flat surface of part 2. FIG. 3 is an enlarged view of the energy director area of the section view of FIG. 2. The tip of the energy director is either sharp or slightly radiused, so as to provide a small contact area between the parts, allowing the ultrasonic vibrations to be focused at the beginning of the weld to initiate melting. The triangular energy director, generally referred to in the industry as simply “energy director,” and its applications are detailed in a number of technical publications, such as the “Handbook of Plastic Joining” (Second Edition, Edited by Michael J. Troughton).
While the triangular energy director design has been in commercial use for many years, several undesirable characteristics are associated with this joint shape. One of the most significant factors is the high manufacturing cost of the part mold containing the energy director. In order to create the sharp edge at the tip, the energy director geometry is typically produced by either electrical discharge machining (EDM) on a single piece of steel, or splitting the mold into two pieces along the center of the energy director with a closely controlled gap between them. The former technique is time-consuming and involves dedicated tooling, and the latter requires tight machining tolerances. Consequently, the cost of mold fabrication is relatively high. Another disadvantage is the challenge in molding parts with energy directors. Specifically, it can be difficult to completely fill the sharp tip with plastic during the molding process. Incomplete or inconsistent energy directors can in turn lead to weaker ultrasonic welds, or welds that are not uniform along the entire joint path. Still another drawback is the susceptibility of the energy director to being damaged prior to welding. After the plastic parts are molded, they are often packaged in bulk and transported to the welding station. If the packaging method permits the energy directors to come in contact with other parts, the energy director rib can be distorted or crushed, sometimes in a highly localized manner. Flaws of this type can result in areas where there is an insufficient amount of material to produce a continuous weld, which is especially problematic in cases where a hermetic seal is required.